1. Field of the Invention
The invention relates to a filter device for the compensation of an asymmetric pupil illumination of an illumination system, especially for an illumination system associated with a lithographic system.
2. Description of the Related Art
High demands are placed on illumination systems for lithographic systems for producing microelectronic or micromechanical components. This relates both to systems which work as wafer stepper or as wafer scanner. Such an illumination system must illuminate an object, which is typically a mask, in a field plane of the illumination system in a homogeneous manner. In addition to this requirement, there is also a demand for the angular distribution of the illumination in the field plane, which on its part is associated with the illumination of the exit pupil of the illumination system. For a lithographic system, the exit pupil of the illumination system coincides with the entrance pupil of a downstream projection objective. That is why it is necessary to arrange the illumination characteristics of the exit pupil in an adjusted manner in order to introduce the largest possible amount of light into the projection objective, to fulfill the requirement of telecentricity in the image plane of the projection system and in order to achieve the most even possible imaging of the mask structures.
For evening the illumination of a field in the field plane, illumination systems are known in which a rod-like optical integrator is used. Depending on the operating wavelength, the material of such a rod-like optical integrator is adjusted. It can consist for example of quartz glass or a crystalline material such as calcium fluoride. The effect of such a rod-like optical integrator is disclosed for example in U.S. Pat. No. 5,675,401, US Pat. No. 2004/012766, EP 0867772, U.S. Pat. No. 6,236,449 or EP 0747772. It provides that as a result of the plurality of total reflections of the light coupled into the rod-like optical integrator, a thorough mixture of the illumination light is achieved on its outside surfaces. The total reflection is not completely without losses due to the residual roughness of the surfaces of the rod jacket.
An undesirable asymmetry of the illumination of the exit pupil occurs in scanners due to the rectangular cross section when using rod-like optical integrators. Light rays which extend predominantly parallel to the narrow side are reflected more frequently and are therefore attenuated more strongly. This asymmetry leads to an energetically elliptic pupil profile and is referred to below as ellipticity. In order to avoid an asymmetry of the illumination, a rod-like optical integrator is known from U.S. Pat. No. 6,733,165 which has such an aspect ratio between width and height, that the number of reflections and thus the total reflection losses on its side surfaces are set in such a way that a predetermined distribution of the luminous energy is produced in the angular space on the output surface of the glass rod. The disadvantageous aspect in this solution according to U.S. Pat. No. 6,733,165 is that only elliptical asymmetries can be corrected.
Furthermore, adjustable symmetric pupil filters are known. U.S. Pat. No. 6,535,274 for example discloses a filter arrangement in which at least two symmetrical filter elements are turned against each other and an adjustable, but symmetrical, intensity filter for filtering the pupil illumination is realized. Pupil filters which are disclosed in U.S. Pat. No. 6,535,274 allow producing or correcting an ellipticity of the distribution of the illumination angle in the object plane by setting the respective transmission in the area of the pupil plane of the illumination system of a projection exposure system. The correction of a complex asymmetry is not possible.
U.S. Pat. No. 6,636,367 shows an illumination system in which changes in the distribution of the angle of illumination can be made through controlled movement of the pupil filter which is arranged in the region of the pupil plane. The pupil filter is arranged as a rotatable element which has a transmission distribution which is non-rotation-symmetric about the rotational axis. Ellipticity can thus also be set in combination with a rod as an integrator.
From US 2003/0076679 an illumination system is known which includes at least one diffraction grating in the light path from the light source to the plane in which the structure-bearing mask is arranged. The diffraction grating is used to reflect light at different angles relative to the optical axis.
Illumination systems have further become known with an optical integrator in the light path from the light source to the plane in which a structure-bearing mask is arranged, e.g. from U.S. Pat. No. 5,731,577, U.S. Pat. No. 5,461,456, U.S. Pat. No. 6,333,777 or EP 0849637.
The optical integrators according to U.S. Pat. No. 5,731,577, U.S. Pat. No. 5,461,456, U.S. Pat. No. 6,333,777 or EP 0849637 includes facetted elements.
Field filters are further known for improving the uniformity of the illumination of a field in the field plane, i.e. filter devices which are positioned closer to a field plane than a pupil plane of the illumination system. EP 1 291 721 discloses a field filter in which the orientation of lamellae-like elements can be set substantially in the ambient environment of the field plane and thus a local blockade effect in the beam path can be achieved. This filter does not allow however correcting the angular spectrum of the illumination of the field plane and thus an asymmetry concerning the intensity of the illumination of the exit pupil of the illumination system.
The disadvantageous aspect in all filter elements known from the state of the art is that they are limited to the correction of certain asymmetries or asymmetric aberrations of the pupil, e.g. to the correction of elliptical asymmetries. The known pupil filters are not suitable for correcting complex asymmetries or asymmetric aberrations in the pupil illumination.
What is needed in the art is a pupil filter with which the disadvantages of the state of the art can be overcome and with which it is especially possible with the pupil filter in accordance with the invention to correct any asymmetry of the illumination of an exit pupil or a pupil conjugated to the exit pupil. This relates especially to illumination systems in which the asymmetries occur in the illumination of the exit pupil which include not only elliptical portions.